NASA Pinpoints Cause of Earth's Recent Record Carbon Dioxide Spike

The last El Nino in 2015-16 impacted the amount of carbon dioxide that Earth's tropical regions released into the atmosphere, leading to Earth's recent record spike in atmospheric carbon dioxide. The effects of the El Nino were different in each region.Credit: NASA-JPL/Caltech› Larger view

A new NASA
study provides space-based evidence that Earth's tropical regions were the
cause of the largest annual increases in atmospheric carbon dioxide
concentration seen in at least 2,000 years.

Scientists
suspected the 2015-16 El Nino -- one of the largest on record -- was
responsible, but exactly how has been a subject of ongoing research. Analyzing
the first 28 months of data from NASA's Orbiting Carbon Observatory-2 (OCO-2)
satellite, researchers conclude impacts of El Nino-related heat and drought
occurring in tropical regions of South America, Africa and Indonesia were
responsible for the record spike in global carbon dioxide. The findings are
published in the journal Science Friday as part of a collection of five research
papers based on OCO-2 data.

"These three
tropical regions released 2.5 gigatons more carbon into the atmosphere than
they did in 2011," said Junjie Liu of NASA's Jet Propulsion Laboratory in
Pasadena, California, who is lead author of the study. "Our analysis shows this
extra carbon dioxide explains the difference in atmospheric carbon dioxide
growth rates between 2011 and the peak years of 2015-16. OCO-2 data allowed us
to quantify how the net exchange of carbon between land and atmosphere in
individual regions is affected during El Nino years." A gigaton is a billion
tons.

In 2015 and
2016, OCO-2 recorded atmospheric carbon dioxide increases that were 50 percent
larger than the average increase seen in recent years preceding these
observations. These measurements are consistent with those made by the National
Oceanic and Atmospheric Administration (NOAA). That increase was about 3 parts
per million of carbon dioxide per year -- or 6.3 gigatons of carbon. In recent
years, the average annual increase has been closer to 2 parts per million of
carbon dioxide per year -- or 4 gigatons of carbon. These record increases
occurred even though emissions from human activities in 2015-16 are estimated
to have remained roughly the same as they were prior to the El Nino, which is a
cyclical warming pattern of ocean circulation in the central and eastern
tropical Pacific Ocean that can affect weather worldwide.

Using OCO-2
data, Liu's team analyzed how Earth's land areas contributed to the record
atmospheric carbon dioxide concentration increases. They found the total amount
of carbon released to the atmosphere from all land areas increased by 3
gigatons in 2015, due to the El Nino. About 80 percent of that amount -- or 2.5
gigatons of carbon -- came from natural processes occurring in tropical forests
in South America, Africa and Indonesia, with each region contributing roughly
the same amount.

The team
compared the 2015 findings to those from a reference year -- 2011 -- using
carbon dioxide data from the Japan Aerospace Exploration Agency's Greenhouse
Gases Observing Satellite (GOSAT). In 2011, weather in the three tropical
regions was normal and the amount of carbon absorbed and released by them was
in balance.

"Understanding
how the carbon cycle in these regions responded to El Nino will enable
scientists to improve carbon cycle models, which should lead to improved
predictions of how our planet may respond to similar conditions in the future,"
said OCO-2 Deputy Project Scientist Annmarie Eldering of JPL. "The team's
findings imply that if future climate brings more or longer droughts, as the
last El Nino did, more carbon dioxide may remain in the atmosphere, leading to
a tendency to further warm Earth."

While the three
tropical regions each released roughly the same amount of carbon dioxide into
the atmosphere, the team found that temperature and rainfall changes influenced
by the El Nino were different in each region, and the natural carbon cycle
responded differently. Liu combined OCO-2 data with other satellite data to
understand details of the natural processes causing each tropical region's
response.

In eastern and
southeastern tropical South America, including the Amazon rainforest, severe
drought spurred by El Nino made 2015 the driest year in the past 30 years.
Temperatures also were higher than normal. These drier and hotter conditions
stressed vegetation and reduced photosynthesis, meaning trees and plants
absorbed less carbon from the atmosphere. The effect was to increase the net
amount of carbon released into the atmosphere.

In contrast, rainfall
in tropical Africa was at normal levels, based on precipitation analysis that
combined satellite measurements and rain gauge data, but ecosystems
endured hotter-than-normal temperatures. Dead trees and
plants decomposed more, resulting in more carbon being released into the
atmosphere. Meanwhile, tropical Asia had the second-driest year in the past 30
years. Its increased carbon release, primarily from Indonesia, was mainly due
to increased peat and forest fires -- also measured by satellite
instruments.

"We knew El Ninos
were one factor in these variations, but until now we didn't understand, at the
scale of these regions, what the most important processes were," said Eldering.
"OCO-2's geographic coverage and data density are allowing us to study each
region separately."

Scott Denning,
professor of atmospheric science at Colorado State University in Fort Collins
and an OCO-2 science team member who was not part of this study, noted that
while scientists have known for decades that El Nino influences the
productivity of tropical forests and, therefore, the forests' net contributions
to atmospheric carbon dioxide, researchers have had very few direct
observations of the effects.

"OCO-2 has
given us two revolutionary new ways to understand the effects of drought and
heat on tropical forests: directly measuring carbon dioxide over these regions
thousands of times a day; and sensing the rate of photosynthesis by detecting
fluorescence from chlorophyll in the trees themselves," said Denning. "We can
use these data to test our understanding of whether the response of tropical
forests is likely to make climate change worse or not."

The
concentration of carbon dioxide in Earth's atmosphere is constantly changing.
It changes from season to season as plants grow and die, with higher
concentrations in the winter and lower amounts in the summer. Annually averaged
atmospheric carbon dioxide concentrations have generally increased year over
year since the early 1800s -- the start of the widespread Industrial
Revolution. Before then, Earth's atmosphere naturally contained about
595 gigatons of carbon in the form of carbon dioxide. Currently, that
number is 850 gigatons.

The annual
increase in atmospheric carbon dioxide levels and the magnitude of the seasonal
cycle are determined by a delicate balance between Earth's atmosphere, ocean
and land. Each year, the ocean, plants and trees take up and release carbon
dioxide. The amount of carbon released into the atmosphere as a result of human
activities also changes each year. On average, Earth's land and ocean remove
about half the carbon dioxide released from human emissions, with the other
half leading to increasing atmospheric concentrations. While
natural processes are responsible for the exchange of carbon dioxide between
the atmosphere, ocean and land, each
year is different. In some years, natural processes remove as little as 20
percent of human emissions, while in other years they scrub as much as 80
percent.

OCO-2, launched
in 2014, gathers global measurements of atmospheric carbon dioxide with the
resolution, precision and coverage needed to understand how this important
greenhouse gas -- the principal human-produced driver of climate change --
moves through the Earth system at regional scales, and how it changes over
time. From its vantage point in space, OCO-2 is able to make roughly 100,000
measurements of atmospheric carbon dioxide each day, around the world.

Institutions
involved in the Liu study include JPL; the National Center for Atmospheric
Research in Boulder, Colorado; the University of Toronto; Colorado State
University; Caltech in Pasadena, California; and Arizona State University in
Tempe.

For more
information on NASA's Orbiting Carbon Observatory-2 mission, visit: